Numerous systems are available today for providing either a continuously variable analog or a digital indication of temperature in which a thermocouple or similar type probe is utilized. The output signal from the thermocouple is processed and a display is provided for giving a visual indication of the temperature. Such a device is shown, for example, in U.S. Pat. No. 3,978,729, and has a digital output which corresponds to the temperature sensed by the probe.
If a digital instrument is utilized to measure temperature and an analog output is required, for example for use with telemetry and analog recording systems, a digital-to-analog converter or similar device customarily is added. The display still provides an indication of the temperature sensed while the converter provides an analog signal which generally varies from a low to a high value as the temperature sensed varies over the complete operating range of the instrument.
Often the range of the temperatures to be monitored lies within only a small portion of the overall range of the instrument. Correspondingly, the analog output variations will be small and difficult to detect. In applications such as telemetry, small variations often lead to errors, and accuracy is lost because the full analog output range is not correlated to the expected range of temperatures to be monitored. A means for easily and reliably adjusting the analog output range is needed that does not affect the accuracy of the instrument and does not require a complicated adjustment procedure or accurate temperature standards for setting the range to correspond to the expected range of temperatures to be monitored. For convenience, the adjustment means should be capable of simulating any temperatures in the range of the instrument.
In accordance with the above, an input circuit is necessary for simulating thermocouple outputs for both the high and low temperatures to be measured while at the same time preventing extraneous thermocouple effects. Relays with low thermocouple effects are available but are relatively expensive.